STI is known as a technique for electrically isolating a device formed on a silicon substrate. An STI process involves etching silicon e.g. with a silicon nitride film as a mask to form trenches, filling an insulating film of e.g. SiO2 into the trenches, and flattening the surface with the mask (silicon nitride film) as a stopper by chemical mechanical polishing (CMP).
When forming trenches in STI, shoulder portions of the trenches (upper corners in the side walls of the grooves) or the corners of the trenches (the corners in the bottoms of the grooves) sometimes take the form of an acute angle. Consequently, a semiconductor device, such as a transistor, is subject to stress concentration in such portions, resulting in the production of defects. The defects may cause an increase in leakage current and an increase in power consumption. Therefore, it is known to form an oxide film on the interior walls of trenches after forming the trenches by etching, thereby rounding off the shape of the trenches.
Thermal oxidation processing using an oxidation furnace or an RTP (rapid thermal process) apparatus is known as a method for forming such a silicon oxide film. For example, in wet oxidation processing, one of thermal oxidation processing methods, which employs an oxidation furnace, a silicon substrate is heated to a temperature exceeding 800° C. and, using a WVG (water vapor generator) apparatus that generates water vapor by the combustion of oxygen and hydrogen, the heated silicon substrate is exposed to an oxidizing atmosphere of water vapor (H2O), thereby oxidizing the silicon surface and forming a silicon oxide film.
Thermal oxidation processing is considered a method capable of forming a good-quality silicon oxide film. Because of the necessity of processing at a high temperature exceeding 800° C., however, the processing method entails the drawbacks of increased thermal budget, distortion of a silicon substrate due to thermal stress, etc.
On the other hand, as a technique which uses a processing temperature around 400° C. and can therefore avoid the drawbacks of increased thermal budget, distortion of a substrate, etc., an oxide film forming method has been proposed which can easily control the thickness of a film and can form a good-quality silicon oxide film by allowing a processing gas to act on the surface of an electronic device, mainly composed of silicon, to carry out oxidation processing using as the processing gas a mixed gas containing argon gas and oxygen gas at an oxygen flow rate ratio of about 1% and using a microwave-excited plasma formed at a pressure of 133.3 Pa in a chamber (e.g. WO2001/69673, WO2004/008519).
This technique, by carrying out plasma processing under the conditions of a processing pressure on the order of 133.3 Pa and an O2 flow rate ratio of 1% in the processing gas (herein referred to as “low-pressure, low-oxygen concentration conditions” for the sake of simplicity), can achieve high oxidation rate and, in addition, has the advantage that when oxidizing a silicon surface having irregularities, a round shape can be introduced into the top corners of the raised portions of silicon, whereby leakage current from the corner portions due to electric field concentration can be suppressed.
However, when a processing object, having a density distribution in a pattern of grooves or a line/space pattern, formed in the surface of the processing object, is subjected to plasma oxidation processing under the above low-pressure, low-oxygen concentration conditions, a difference in the rate of forming a silicon oxide film will be produced between a nondensely-patterned area and a densely-patterned area, making it impossible to form a silicon oxide film with a uniform thickness. A silicon oxide film whose thickness varies depending on such areas, when used as an insulating film of a semiconductor device, will lower the reliability of the semiconductor device.
As semiconductor devices are becoming increasingly miniaturized, it is desirable to enhance the selectivity of the thickness of an oxide film formed on the bottoms of trenches over that of the film formed on the side walls of the trenches so as to make the oxide film formed on the side walls of the trenches thinner.